1. Field of the Invention
The present invention relates to an intermediate transfer medium and an image forming device, both to be utilized for an image forming method utilizing a process comprising primarily transferring a toner image formed on a photoreceptor onto the intermediate transfer medium and transferring and fixing the primarily transferred toner image on recording paper, and a method for producing the intermediate transfer medium.
2. Description of the Background Art
The imaging technique comprising forming an electrostatic latent image on a photoreceptor, developing the image as a toner image by means of dry toner, and thereafter transferring and fixing in an electrostatic manner the toner image on a recording medium has been used conventionally in a wide variety of fields, but the technique is disadvantageous in that the resulting image is uneven from the respect of density or powder toner is dispersed on the transfer part, with the resulting deterioration of resolution and dot reproducibility.
The reason principally lies in the process of transferring in an electrostatic manner the toner image on the photoreceptor onto a recording medium.
Because the transfer efficiency of a toner layer depends on Et, the efficiency varies depending on the thickness of the toner layer and the transfer position on a recording medium, unless Et is maintained constant irrespective of the recesses and protrusions of the recording medium representatively illustrated by paper and the electrical properties thereof. When the toner image formed on the photoreceptor is of a monochromatic layer of a thin thickness, the unevenness in the resulting image is principally induced by the recesses and protrusions of the recording medium and the electrical properties thereof. This is the case when monochromatic toner images individually formed on a plurality of photoreceptors are overlaid and transferred onto a single recording medium to form a color image, and therefore, the recesses and protrusions of a recording medium and the electrical properties thereof cause unevenness in the resulting image. In other words, the difference between overlaying and transferring parts and monochromatic transfer parts without overlaying or transferring can be suppressed in an electrostatic manner, but it is difficult to supplement the recesses and protrusions of the recording medium or the unevenness of the electrical properties thereof.
Alternatively, an electrostatic transfer process has been known, comprising overlaying and transferring monochromatic toner images individually formed on a plurality of photoreceptors onto an intermediate transfer medium under controls of the properties without recess or protrusion, so-called intermediate transfer medium (for example, belt and drum), and thereafter transferring the resulting color image on a recording medium. In this case, a uniform image without unevenness can be formed on the intermediate transfer medium. However, the toner images on the intermediate transfer medium are of multiple layers, namely three layers or more at parts with abundant layers and one layer or less at parts with lesser layers, and therefore, it is difficult to load a constant electric field onto these toner layers so as to integrally transfer the layers uniformly in an electrostatic manner onto a recording medium representatively illustrated by paper. Consequently, the resulting Et is non-uniform. By such electrostatic transfer process, thus, not the whole toner image color overlaid on the intermediate transfer medium is transferred, but the image may partially remain on the intermediate transfer medium. Additionally, the extent of the image left thereon varies depending on the thickness of the toner layers formed on the intermediate transfer medium. Therefore, the resulting color image on the recording medium loses color balance, so that a desired color image cannot be recovered. Additionally, recording media such as paper cannot closely adhere to the intermediate transfer medium due to the presence of the recesses and protrusions on the surface thereof, involving the occurrence of non-uniform gaps, which causes the disorder of the transfer electric field or the coulomb repulsion of the toner powder from each other to disperse the toner powder. Then, the quality of the resulting image is deteriorated.
For the countermeasure against the problem, Japanese Published Examined Patent Application No. Sho 46-41679 discloses a method comprising adhesion transferring a toner image formed on a photoreceptor onto an intermediate transfer medium and subsequently thermally transferring the toner from the intermediate transfer medium onto a recording medium. Because the transfer of the toner image onto the recording medium progresses in a non-electrostatic manner by the method, the deterioration of the image quality at the electrostatic transfer process as described above hardly occurs.
Furthermore, Japanese Unexamined Patent Application No. Hei 2-108072 discloses a technique comprising overlaying and transferring toner images of different colors onto an intermediate transfer medium, fusing the multiple toner images of multiple colors on the intermediate transfer medium and transferring the fused multiple toner images onto a recording medium. Because the transfer of the toner images onto the recording medium progresses in a non-electrostatic manner by the method, the deterioration of the image quality as described above hardly occurs (the method is called "transfer fixing method" hereinafter).
As to the image forming device utilizing the transfer fixing method, U.S. Pat No. 2,990,278, Japanese Published Unexamined Patent Application No. Hei 5-19642, Japanese Published Unexamined Patent Application No. Hei 5-107950 and Japanese Patent Laid-open No. Hei 5-249798 disclose a technique comprising keeping an intermediate transfer medium and a recording medium closely together for heating and pressing, so as to completely transfer the toner image from the intermediate transfer medium to the recording medium, cooling the toner image until the cohesion potency of the toner is above the adhesion potency between the toner and the intermediate transfer medium (at least below the melting point of the toner), and peeling off the recording medium from the intermediate transfer medium. According to the technique, an image of high quality including high toner transfer efficiency, good color balance, high gloss and good toner transparency, can be generated. So as to effectively utilize the advantages, subsequently, research works have been made regarding the composition of the uppermost layer of such intermediate transfer medium, in particular.
For example, investigations have been made about silicone rubber, fluorine resins or fluorine resins dispersed in fluorine rubber, as materials with thermal resistance and toner peelability.
Unlike conventional fixing devices, devices utilizing an intermediate transfer medium cannot feed releasing agents such as silicone oil onto the surface of the intermediate transfer medium because the intermediate transfer medium is in contact to the surface of a photoreceptor. For the purpose of preventing the offset phenomenon of a toner image on the surface of an intermediate transfer medium during transfer and fixing to reduce the defects of the resulting image, therefore, silicone rubber with advantages such as good releasability from toner is frequently used as a surface material of such intermediate transfer medium.
However, the following problems occur even when an intermediate transfer medium coated with silicone rubber is used.
On the surface of a photoreceptor are present a great number of toners called as fog toners, other than toner images forming an image. For directly transferring toner images on the surface of a photoreceptor on a recording medium by utilizing electrostatic power, fog toners are selectively transferred by utilizing electrostatic power, so the image of the fog toners is at a non-detectable level on the recording medium even if transferred thereon. However, almost all of the fog toners are transferred on the surface of the aforementioned intermediate transfer medium, if it is used, because of the elasticity and adhesiveness of the silicone rubber. Consequently, the resulting image quality is essentially deteriorated, disadvantageously.
When toner images once melt are cooled below the melting point of toner and are then released, the toner images released from the surface of the intermediate transfer medium follow the surface profile of the intermediate transfer medium on the recording medium, although no offset phenomenon onto the surface of the intermediate transfer medium may occur. More specifically, the surface of the toner images approximately follows the surface profile of the intermediate transfer medium as if by molding, characteristically; if the surface of the intermediate transfer medium is smooth, the toner images are so glossy, namely with a higher gloss, while the toner images are at a lower gloss if the surface of the intermediate transfer medium is rough or foggy. Therefore, the surface of silicone rubber should essentially be maintained at a smooth state if a higher gloss is required. This is readily attained because silicone rubber has greater leveling property. However, another problem occurs in such case. In other words, the following problem occurs, concerning the fact that because toner images of three colors or more should be overlaid together for development so as to copy and print a color image, the shift of individual registrations, namely positional shift, is of great significance on image quality.
Intermediate transfer mediums of smooth surface, such as mediums coated with silicone rubber, have higher frictional coefficients with very smooth surface of photoreceptors and the like. Consequently, slipping occurs between the driving part (for example, driving roll) of the intermediate transfer medium and the back face of the intermediate transfer medium. When the frictional coefficient between the driving roll and the back face of the intermediate transfer medium is raised to increase the driving force, the intermediate transfer medium waves or cannot retain its planar surface because the direction of the force supplied from the driving roll does not strictly agree with the direction of the force supplied from the photoreceptor, generally due to mechanical precision, and these powers work to pull the intermediate transfer medium together. No problem occurs when these directions agree with each other. Consequently, the toner images on the surface of the photoreceptor cannot be transferred accurately, disadvantageously involving the occurrence of image defects.
So as to reduce the frictional coefficient of rubber, generally, a method comprising roughing the surface is used. A number of such methods have been known. One of the methods comprises roughing the surface through spray coating under conditions hardly fogging silicone rubber in the spray, for example, by modifying coating conditions such as temperature, moisture and the distance from a spray gun and by modifying the viscosity of silicone rubber. By the method, however, fine undulations can be made but the surface thereof remains smooth, so the frictional coefficient of the rubber cannot be reduced so much. Another method comprises a blast process of blasting sand or steel particles, which roughs the overall rubber surface by preparing recess parts on the surface. However, the frictional coefficient is only slightly decreased, or the transfer rate of the fog toner cannot be reduced. The gloss of the resulting image is severely reduced, and additionally, the rubber can be roughed uniformly with much difficulty, which causes the surface uneven. Hence, the resulting image quality is severely deteriorated. Blade coaters and dipping can be used as other coating means, but both means make the surface like mirror face even though these means can generate greater undulations. Accordingly, the problems to be overcome and focused by the present inventors can never be solved.
Some of prior art include the description about the surface roughness of intermediate transfer mediums. Japanese Published Unexamined Patent Application No. Sho 59-50473 describes that the surface roughness can be controlled by spray coating, but according to the description, the frictional coefficient of the surface with a photoreceptor cannot be reduced enough to control and slide the intermediate transfer medium on the photoreceptor or at such a level that not any transferred fog toner is detectable. Besides, Japanese Published Unexamined Patent Application Nos. Sho 59-202477, Hei 5-19642, Hei 5-333711, Hei 6-102782, Hei 7-43992, and Hei 8-185061 of the prior art only define the surface roughness of intermediate transfer mediums, for the purpose of improving the durability of the rubber on the surface of the intermediate transfer mediums, the transferability thereof, and image gloss, preventing transfer error, and improving the adhesiveness with transferring materials, for the purpose of preventing toner offset on the surface of intermediate transfer mediums, and for the purpose of improving the shelf life of releasing agents so as to prevent the adverse effects of the releasing agents such as oil. Even if the surface roughness as disclosed in these publications is attained, not all of the following requirements can be satisfied; high gloss desirable for color images with no observable effect of the recesses and protrusions on the surface of the intermediate transfer mediums on the images; good control of driving run of the intermediate transfer mediums for preventing image shift (namely, low frictional coefficient with photoreceptors) and reduction of the transfer rate of fog toner.